Remove the overflowchanged event with a runtime flag

Remove the overflowchanged event with a runtime flag

Intent to Remove:
https://groups.google.com/a/chromium.org/d/msg/blink-dev/NZ7RGtrPQsw/DRQmRzSkGNAJ

A full removal touches many files, so keep it behind a flag in case this
needs to be reverted.

The tests are much less prone to conflicts, so in order to avoid
creating a virtual testsuite, simply remove them. They can be restored
if necessary later.

All but one of the removed tests are explictly for some aspect of the
overflowchanged event. The exception is
layout-overflow-not-affected-by-visual-overflow.html which is from the
fix of "RenderBox layout overflow is not correctly computed":
https://code.google.com/p/chromium/issues/detail?id=254002

Unfortunately, it does not appear that the bug has any other observable
effect that can be used. An old version of Chromium with the bug was
tested; the rendered result is identical and overflow:scroll does not
cause any scrollbars to appear. Therefore, the test is removed.

BUG=460822
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=190938

[philipj_slow, 2015-02-26 11:41:15]

tests

[philipj_slow, 2015-02-26 13:51:59]

philipj@opera.com changed reviewers:
+ chrishtr@chromium.org

[philipj_slow, 2015-02-26 13:52:00]

PTAL

[chrishtr, 2015-02-26 17:45:02]

The CQ bit was checked by chrishtr@chromium.org

[chrishtr, 2015-02-26 17:45:03]

lgtm

[commit-bot: I haz the power, 2015-02-26 17:45:23]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/959643002/20001

[commit-bot: I haz the power, 2015-02-26 17:49:08]

Committed patchset #2 (id:20001) as
https://src.chromium.org/viewvc/blink?view=rev&revision=190938